Social request for environment protection, such as reduction of carbon dioxide emission, has increased a demand for automobile fuel efficiency.
In such a present situation, development of materials exhibiting low rolling resistance for automobile tires, particularly for tire tread which directly contacts with the ground, has been demanded.
On the other hand, from the viewpoint of the safety, materials which are excellent in wet skid resistance and which also have abrasion resistance and strength sufficient for practical use are required.
As reinforcing fillers for tire treads, carbon black, silica and the like are conventionally used.
Use of silica as a reinforcing filler has an advantage of achieving low hysteresis loss and improvement in the wet skid resistance.
On the other hand, in contrast to the hydrophobic nature of carbon black surface, silica surface is hydrophilic and has low affinity for conjugated diene-based rubbers, which causes poorer dispersibility of silica in conjugated diene-based rubber compounds than the carbon black. In order to improve the dispersibility and impart bonding between the silica and the rubbers, a silane coupling agent needs to be added.
In consideration of such problems with silica, attempts of introducing functional groups having affinity and/or reactivity to silica to the polymer chain end have been made. By introducing functional groups to the chain ends which have high mobility, the hysteresis loss can be reduced by improving the dispersibility of silica in conjugated diene-based rubber compounds and further, by reducing the number of free polymer chain ends via interaction with the filler particles.
For example, there are proposed a modified diene-based rubber obtained by reacting a modifier having a glycidylamino group with a polymer chain end (for example, see Patent Document 1), a modified diene-based rubber obtained by reacting glycidoxyalkoxysilane with a polymer chain end (for example, see Patent Document 2), further a modified diene-based rubber obtained by reacting an alkoxysilane containing an amino group with a polymer chain end (for example, see Patent Documents 3 and 4), and compositions thereof with silica.
A technology is also proposed in which a diene-based rubber is polymerized using a polyfunctional anionic polymerization initiator, and thereafter is modified with a modifier such as a glycidylamino group to increase the number of functionalized polymer chain ends, thereby improving the performance of a composition constituted of the diene-based rubber and silica, that is, the silica dispersibility, and reducing the hysteresis loss (for example, see Patent Document 5).